The present invention relates to a method of and an apparatus for processing an image signal, and more particularly to a method of and an apparatus for processing an image signal such that when image information containing a regular or periodic pattern is to be processed for halftone dot formation, the image information is first processed for unsharpness and then processed for sharpness, thereby reproducing a sharp image free from a moire pattern.
Image scanning reading and reproducing systems are widely used in the printing and platemaking industries for electrically processing image information of originals or subjects to produce original film plates with a view to simplifying the entire process and improving the quality of printed images.
The image scanning reading and reproducing systems are basically constructed of an image reading apparatus and an image recording apparatus. In the image reading apparatus, image information of an original or subject which is fed in an auxiliary scanning direction is scanned by a photosensor in a main scanning direction substantially normal to the auxiliary scanning direction, and the scanned image information is converted to an electric signal. Then, the photoelectrically converted image information is processed according to platemaking conditions. Thereafter, the processed image signal is converted into a light signal such as a laser beam signal which is applied to and recorded on an image recording medium of a photosensitive material such as a photographic film in the image recording apparatus. The image recording medium with the image recorded thereon is developed by an image developing device and will be used as a film plate for printing.
In the image scanning and reproducing systems, a continuous-tone image is converted into a halftone-dot image in order to reproduce the gradations of the continuous-tone image. If the image of an original has a regular or periodic pattern such as on a printed halftone-dot image, then the periodic pattern is stressed and the reproduced halftone-dot image tends to have a moire pattern.
One solution to the above problem is to adjust the angle of the halftone screen used to break up a continuous-tone image into halftone dots so that the generation of any moire pattern will be minimized. However, selecting an optimum halftone screen angle is a tedious and time-consuming task itself and also requires skill on the part of the operator. Another proposal involves alteration of the aperture size of the halftone screen for blurring the image, so that any periodic pattern contained in the image will be less recognizable. This process is however disadvantageous in that the processed image is relatively unclear and hence poor in quality. In addition, the process can be used in a limited range of applications since it is effective with only a system in which the aperture size is physically alterable.